The Union of States that now exist was created in 1959 and by the changes to Constitution since then. The Union of 1787 was changed every time an amendment was added, and when another State was added to the group- That union died in 1860- The Constitution was ignored its own government and by 1866 the United States became centralized nation created by the period of State reconstuction. What now exist has little in common with the union that existed when the U.S. Constitution was ratified.
The use of the word "union" dropped out of favor and is only used by 19th Century historians. Prior to the Civil War the word was highly cherished by most Americans until the Union was driven to destruction by the flaws of the original U.S. Constitution and the process in which it was created. The U.S. Constitution totally failed to live up to its decree to "form a more perfect union" after 73 years of existance. The Civil War proved the Constitution had/has very little power in stopping States or the Federal Government from coercing States, so one of its fundamentals, protection and security failed.
"Therefore if that Constitution had not been agreeable in its terms and limitation placed upon the central government under that Constitution to those states, there would not have been a Union at all."
Had some of the original framers of the Constitution had their way there would be no individual States to form a Union- just one big nation. If the U.S. Constitution is more valuable than the Union then this idea would have been more popular when it was being written forming one complete nation under one government instead of the thirteen individual States, their people and their own Constitutions and governments that ratified the Constitution. The U.S. Constitution does not define the States the States define the Constitution.
Those States that did not immediately ratify the U.S. Constitution were in a legal Union under the old Articles. The Supreme Court ruled that they were years later. There had always been a union of Colonies/States of some sort since the early 17th Century in English America. Had the U.S. Constitution failed to be ratified, there most probably would have been a different government formed between the States, but there would have been some sort of union. As long as Great Britain had any memory or desires about their former colonies-- there would be a United States- as there was in 1776 and would be proven in 1815; where it was not the U.S. Constitution that kept the union together but the United States and its people.